Polyethylene (PE) is synthesized via polymerizing ethylene (CH2═CH2) monomer and optionally a higher 1-olefin comonomer such as 1-butene, 1-hexene, 1-octene or 1-decene. Because PE is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the synthesis methods, PE can be generally classified into several types such as LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
For polyolefins, such as PE polymers and/or co-polymers, the molecular weight distribution (MWD) of the polymer particles is one of the basic properties that determine the characteristics of the polymer resin, and thus its end-use applications. The MWD of a polymer may be described by a graphical representation of the molecular weight composition of the material obtained through analysis, for example, by gel permeation chromatography, however, the MWD can also be described by the polydispersity index D, which is the ratio Mw/Mn of the weight average molecular weight Mw to the number average molecular weight Mn.
An increase in the molecular weight of a polyolefin resin can improve certain physical properties of the resin, however, high molecular weights can also tend to make polymers more difficult to process. A polymer having a high molecular weight will typically be more difficult to melt and flow, which can be referred to as having a relatively low melt flow index (MI). An increase in the MWD of a resin can tend to improve the ability of the polymer to flow during the processing, for example, by increasing the quantity of a lower molecular weight polymer portion of the resin in relation to the higher molecular weight polymer portion of the resin. Thus, broadening the MWD is one way to improve the processing of a high molecular weight polyolefin. This can be particularly useful in applications requiring relatively fast processing, such as in some blow molding and extrusion techniques. An increase in processability of the polymer can facilitate higher processing throughput rates and lower energy requirements.
When two or more polymers having differing molecular weight characteristics are combined, the resulting mixed polymer can comprise a broadened MWD and can also comprise a multimodal molecular weight distribution. A multimodal MWD can be described as the summation of the MWD of the individual polymers being combined, which can in some embodiments result in a MWD comprising multiple molecular weight ranges having increased concentration. A typical bimodal MWD will comprise two areas of concentration within the overall molecular weight range of the polymer, often referred to as the high molecular weight HMW fraction and the low molecular weight LMW fraction. Benefits of a multi-modal MWD can include, for example, improved physical properties obtained from the HMW fraction, and improved processing capabilities obtained from the LMW fraction.
Mechanical efforts have been used in an effort to prepare resins having broad and/or bimodal MWD by blending polyolefin portions having different molecular weights, however, the results of mechanical blends are limited by the degree of physical mixing that is possible and the size of the particles being mixed, typically in a pellet type form. Mechanical means do not result in the mixing of various polyolefin pellets on a microscopic scale, and therefore will not behave like an intimate blend of polyolefins that are prepared in-situ within a common polymerization process.
In a typical polymerization reaction, monomer, diluent and a dry particulate catalyst are fed to a reactor where the monomer is polymerized. The diluent does not react but is typically utilized to control solids concentration and also to provide a convenient mechanism for introducing the catalyst into the reactor. The reactor effluent, a mixture of polymer, diluent and unreacted monomer, is removed from the reactor and fed to a flash tank where the polymer is separated from the diluent and unreacted monomer. Typically, catalyst will be contained in the polymer.
The use of metallocene catalysts in the production of polyolefins in general, and of polyethylene in particular, is known in the art.
In general, for preparing catalyst slurry, a mixture of dry solid particulate catalyst and diluent are apportioned in a catalyst storage vessel for thorough mixing. Then such catalyst slurry is typically transferred directly to a polymerization reaction vessel for contact with the monomer reactants, generally under high pressure conditions. However, it is important to control catalyst flow to a reactor since unexpected or uncontrolled catalyst injection in a reactor could lead to runaway reactions. Direct feeding of catalyst slurry from a storage vessel to a reactor has the disadvantage that the feeding rate of the catalyst to the reactor cannot be adequately controlled. Also, in cases involving direct feeding of a catalyst from a mud pot to a reactor, the metallocene catalyst can be completely flushed in the reactor, when a problem occurs during the preparation of the metallocene catalyst. Such uncontrolled catalyst feeding may induce runaway reactions in the reactor.
Improvements in the feeding of catalyst to a reactor have been described, e.g. in U.S. Pat. No. 5,098,667. This US patent describes a method for feeding of a catalyst in general to a reactor comprising preparing heavy slurry in a storage vessel, and then transferring the heavy slurry to a mixing vessel, where the heavy slurry is diluted and subsequently transferred to a reactor. In the described method the flow rate of the dilute slurry is manipulated so as to provide a desired flow rate of solid particles contained in the dilute slurry. Continuous catalyst flow is maintained at a desired rate in response to a computed value of the mass flow rate of the solid catalyst particles contained in the dilute slurry. The computed mass flow rate of catalyst particles is based upon “on line” measurements of density and flow rate of the dilute catalyst slurry stream flowing to the reactor, and on predetermined densities of the solid catalyst particles and the liquid diluent constituting the slurry. However, although the method provides an improvement on the control of catalyst flow, it has the disadvantage that the catalyst flow rate is not adjusted in function of the reaction conditions in the polymerization reactor.
Therefore, there remains a need in the art for providing an improved method for controlling catalyst feeding, and in particular feeding of metallocene or chromium catalysts, to a polymerization reactor.
Furthermore, metallocene catalysts are usually employed with a co-catalyst for olefin polymerization, which can significantly enhance the polymerization efficiencies to beyond a million units of polymer per unit of catalyst. The co-catalyst is an organometallic compound, or a mixture of non-coordinated Lewis acid and alkylaluminium as it is well known in the art. A number of techniques for the introduction of the co-catalyst to a polymerization reactor has been proposed. For instance some techniques consist of introducing the co-catalyst directly into the polymerization reactor. However, such technique does not allow bringing the co-catalyst into contact with the metallocene catalyst before entering the reactor, although such pre-contact is particularly desirable in order to provide effective catalyst-co-catalyst mixtures. Another technique consists of contacting the catalyst and co-catalyst before their introduction into the polymerization medium. In this latter case, however, having regard to the fact that the catalyst systems employed usually have maximum activity at the commencement of polymerization, it may be difficult to avoid reaction runaways liable to involve the formation of hot spots and of agglomerates of molten polymer.
In view hereof, it can be concluded that there remains also a need in the art for providing an improved method for controlling catalyst feeding, in particular feeding of metallocene catalysts, in pre-contact with a co-catalyst, to a polymerization reactor.
It is therefore a general object of the present invention to provide an improved apparatus and method for feeding catalyst to a polymerization reactor, at a controlled flow rate. Another object of the present invention is to provide an apparatus and method for controlling the injection of catalyst slurry, in particular metallocene or chromium catalyst slurry, in a polymerization reactor, wherein polyethylene is prepared.
It is a further object of the present invention to provide an apparatus and method for controlling catalyst feeding, and in particular feeding of a metallocene catalyst, being in pre-contact with a co-catalyst, to a polymerization reactor, wherein polyethylene is prepared.
Furthermore, the present invention aims to provide a method for improved control of the polymerization reaction of ethylene in a reactor.
Polyethylene polymerizations are frequently carried out using monomer, diluent and catalyst and optionally co-monomers and hydrogen in a loop reactor. The polymerization is usually performed under slurry conditions, wherein the product usually consists of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized.
Ethylene co-polymerization is the process wherein ethylene is polymerized with an olefin co-monomer, such as e.g. propylene, butene, hexene, etc. A major problem in such co-polymerization process is that the control of reaction parameters is very difficult. In particular, the ratio of co-monomer to monomer (ethylene) differs at different points in the reactor.
As a result of the variation in the co-monomer/ethylene ratio throughout the reactor, reaction conditions will vary along the path of the polymerization reactor. As the monomer (ethylene) is depleted faster than the co-monomer in the reactor, fluctuations in reaction temperatures and fluctuations in monomer concentration along the reactor occur. In addition, due to varying reaction conditions in the reactor, the polymerization reaction is sub-optimal and polymer particles will be obtained during the polymerization process, which have varying properties and have a non-homogenous composition. In certain cases, due to the variation in the co-monomer/ethylene ratio throughout the reactor, polyethylene is produced having a too low density, which could induce “swelling” of the polymer particles. Swelling refers to the process whereby formed polymer particles are dissolved in diluent, giving rise to polymer slurry which is more viscous, which has undesired properties, and which may block the polymerization reactor.
In view hereof, it is a need in the art to provide a process for improving the co-polymerization reaction of ethylene with an olefin co-monomer, such that the co-polymerization process is optimized and that more homogenous polymer end products are obtained.
It is therefore an object of the present invention to provide a process for improving the co-polymerization of ethylene and an olefin co-monomer. It is in particular an object of the invention to provide a process for controlling the co-monomer/ethylene ratio in a polymerization reactor. The present invention aims to provide a process for obtaining a co-polymer end product having improved compositional homogeneity and improved quality.
It is known that the polymerization of olefins, e.g. ethylene, involves the polymerization of olefin monomer with the aid of an organometallic catalyst of Ziegler-Natta and a co-catalyst. Catalyst systems for polymerization and co-polymerization of olefins known as Ziegler-Natta systems consist on the one hand, as catalyst, of compounds of transition metals belonging to Groups IV to VII of the periodic table of elements, and on the other hand, as co-catalysts, of organometallic compounds of metals of Groups I to III of this Table. The catalysts most frequently used are the halogenated derivatives of titanium and vanadium, preferably associated with compounds of magnesium. Moreover, the co-catalysts most frequently used are organoaluminium or organozinc compounds. When the catalyst is highly active, especially when it is employed in the presence of a large quantity of co-catalyst, a formation of polymer agglomerates, which may be considerable, can be observed. In a typical Ziegler-Natta catalysis system the monomer, e.g. ethylene or propylene, is bubbled into the suspended catalyst and the ethylene or propylene rapidly polymerizes to a high molecular weight linear polyethylene or polypropylene. A characteristic of all Ziegler-Natta catalysts is that they all yield straight chain polymers.
The use of Ziegler-Natta-catalysts in a polymerization method has been improved over a number of generations since the initial work by Ziegler and Natta in the 1950s. Seeking to increase both the activity and the stereoselectivity has been the driving force for the continuous development of the catalyst system. In addition to the support material, this comprises as actual catalyst a transition metal compound, e.g. a titanium compound, which is activated only by addition of an aluminium-containing co-catalyst.
It is known that the activity of certain Ziegler catalyst systems can be improved by increasing the quantity of organometallic compound used as the co-catalyst. In this case, it is generally necessary to employ in the polymerization medium relatively large quantities of organometallic compounds as co-catalysts. However, this provides disadvantages including safety problems, related to the fact that these organometallic compounds spontaneously ignite on contact with air.
In employing Ziegler-Natta catalysts, it has been customary to inject the catalyst as a slurry in a diluent into a reaction zone of the reactor and to introduce also the olefins being polymerized. Several methods for supplying catalyst to a polymerization reactor have been described in the prior art.
U.S. Pat. No. 3,846,394 describes a process for the introduction of Ziegler-Natta catalyst slurry in a reactor. The process comprises the preparation of Ziegler-Natta catalyst slurry, the transfer of the slurry via a feed conduit from a storage zone to a metering zone, and the introduction of the slurry into a reactor. In order to avoid the back flow of monomer and other contents of the reactor into the Ziegler-Natta catalyst conduits the process provides the catalyst feed conduit to be flushed with a diluent inert to the Ziegler-Natta catalyst, said diluent being introduced into said conduit downstream of the metering zone.
It is well known that the polymerization reaction is quite sensitive to the quantity of catalyst utilized, and it is also known that the amount of catalyst added to the reactor is based on the flow rate of the catalyst to the reactor. However, one of the major problems in the injection of Ziegler-Natta catalyst slurry in a diluent to a reactor in prior art methods is that it is difficult to control the amount of Ziegler-Natta catalyst injected. Also, the catalyst tends to clog catalyst injection means such as pumps and the like and lines carrying the slurry.
For instance, U.S. Pat. No. 3,726,845 describes the supply and control of the amount of catalyst and the maintenance of the catalyst line and pump free by alternately feeding catalyst slurry and diluent to the reaction zone, allowing careful control of the amount of catalyst and control of the cleanliness of equipment such as lines and pumps and freedom from clogging.
GB 838,395 relates to a process and apparatus for producing a slurry of a solid catalyst in hydrocarbon diluent for use in a chemical reaction. The process comprises preparing concentrated catalyst slurry in a hydrocarbon diluent and admixing said concentrated slurry with additional diluent and introducing said admixture to a reaction zone. According to the process, the specific inductive capacity of the slurry is continuously determined prior to the introduction of same to said reaction zone, the inductive capacity of the slurry being dependent upon the concentration of catalyst in the slurry. The process further comprises regulating the ratio of concentrated slurry to added diluent responsive to variations of said specific inductive capacity from a predetermined value so as to maintain a slurry of substantially constant dielectric value.
Moreover, another problem relates to catalyst supply is that it has been difficult to control Ziegler-Natta catalyst flow rate in an adequate way. Ziegler-Natta catalyst flow rate is generally fixed for a certain operation and catalyst feeding systems do not account for variations in the feed flow rate.
Another problem relating to the field of catalyst supply to a reactor consists of supplying a co-catalyst during a polymerization reaction. A number of techniques for the introduction of the co-catalyst has already been proposed, for example by introducing the co-catalyst directly into the polymerization reactor. However, such methods do not allow bringing co-catalyst into contact with the Ziegler-Natta catalyst before entering the reactor, although such pre-contact is particularly desirable in order to provide effective Ziegler-Natta catalyst-co-catalyst mixtures.
Another technique consists of contacting the catalyst and co-catalyst before their introduction into the polymerization medium. In this latter case, however, it is difficult to control the pre-contact time of the catalyst with the co-catalyst.
It is therefore a general object of this invention to provide an improved method for optimising catalyst introduction in a polymerization reactor. It is an object of the present invention to optimise the supply of a Ziegler-Natta catalyst to a polymerization reactor wherein polyethylene is prepared. More in particular, the present invention also aims to provide a method enabling to effectively control the flow rate of a catalyst, and in particular a Ziegler-Natta catalyst, to a polymerization reactor wherein polyethylene is prepared.
It is another object the present invention to provide a method for supplying catalyst, and in particular a Ziegler-Natta catalyst, in pre-contact with a co-catalyst, to a polymerization reactor, wherein polyethylene is prepared.
Furthermore, the present invention aims to provide a device for preparing catalyst slurry, in particular a Ziegler-Natta catalyst, and for supplying said catalyst slurry to a polymerization reactor in a controlled and efficient way.
It is known that the polymerization of olefins e.g. ethylene, especially by a gas phase polymerization process, involves the polymerization of olefin monomer with the aid of catalyst and optionally, if required depending on the used catalyst, a co-catalyst. Suitable catalysts for use in the production of polyolefins, and in particular for the preparation of polyethylene, comprise chromium-type catalysts, Ziegler-Natta catalysts and metallocene catalysts.
It is well known that the polymerization reaction is quite sensitive to the quantity of catalyst utilized. It is important to control catalyst flow to a reactor since unexpected or uncontrolled catalyst injection in a reactor could lead to runaway reactions. However, one of the major problems in the injection of catalyst slurry to a reactor in prior art methods is that it is difficult to control the amount of catalyst and the flow rate of the catalyst injected.
GB 838,395 relates to a process and apparatus for producing a slurry of a solid catalyst in hydrocarbon diluent for use in a chemical reaction. The process comprises preparing concentrated catalyst slurry in a hydrocarbon diluent and admixing said concentrated slurry with additional diluent and introducing said admixture to a reaction zone. According to the process, the specific inductive capacity of the slurry is continuously determined prior to the introduction of same to said reaction zone, the inductive capacity of the slurry being dependent upon the concentration of catalyst in the slurry.
U.S. Pat. No. 5,098,667 describes a method for supply of a catalyst in general to a reactor comprising preparing heavy slurry in a storage vessel, and then transferring the heavy slurry to a mixing vessel e.g. by means of a metering valve such as a ball check valve, where the heavy slurry is diluted and subsequently transferred to a reactor. In the described method the flow rate of the diluted slurry is manipulated so as to provide a desired flow rate of solid particles contained in the diluted slurry. Continuous catalyst flow is maintained at a desired rate in response to a computed value of the mass flow rate of the solid catalyst particles contained in the dilute slurry. The computed mass flow rate of catalyst particles is based upon “on line” measurements of density and flow rate of the dilute catalyst slurry stream flowing to the reactor, and on predetermined densities of the solid catalyst particles and the liquid diluent constituting the slurry.
However, although the above-described methods provide an improvement on the control of catalyst flow, they have the disadvantage that the catalyst flow rate can not be reliably adjusted in function of the reaction conditions in the polymerization reactor.
Furthermore, direct feeding of catalyst slurry to a reactor has the disadvantage that the feeding rate of the catalyst to the reactor cannot be adequately controlled. Also, in cases involving direct supply of a catalyst to a reactor, the catalysts can completely be flushed in the reactor, when a problem occurs during the preparation of the catalysts. Such uncontrolled catalyst supply may induce runaway reactions in the reactor.
Moreover, in the case catalyst in oil suspension is provided directly to a reactor, the used pumps, generally progressive cavity pumps, are not able to dose the catalyst flow and the amount of catalyst injected in the reactor. Furthermore, such systems require the switch over of the catalyst injection system, every time a new batch of catalyst needs to be connected to the reactor for supply thereto. Therefore, such injection systems do not provide an optimal and reliable control of the catalyst flow rate.
In view hereof, it can be concluded that there remains a need in the art for providing an improved method for controlling catalyst supply to a polymerization reactor.
It is therefore a general object of this invention to provide an improved method for optimising catalyst introduction in a polymerization reactor. It is a particular object of the present invention to optimise the supply of a catalyst, commercially provided in an oil suspension, to a polymerization reactor wherein polyethylene is prepared. More in particular, the present invention also aims to provide a method enabling to effectively control the flow rate of a catalyst to a polymerization reactor wherein polyethylene is prepared.
Furthermore, the present invention aims to provide an apparatus for preparing catalyst slurry, and for supplying said catalyst slurry to a polymerization reactor in a controlled and efficient way.
Olefin polymerization processes are well known. Among the processes, slurry polymerization in suspension in a solvent or in the liquid monomer is extensively practiced. Such processes are performed in a stirred tank reactor, or in closed loop reactors. One or more reactors can be used. In such processes, solid polymer particles are grown on small catalyst particles. Released heat of polymerization is eliminated through cooling through the reator's walls and/or a heat exchanger.
However, it has been found on an industrial scale that while the polymer particles are insoluble or substantially insoluble in the diluent, the polymer product has some tendency to deposit on the walls of the polymerization reactor. This so-called “fouling” leads to a decrease in the efficiency of heat exchange between the reactor bulk and the coolant around the reactor. This leads in some cases to loss of reactor control due to overheating, or to reactor or downstream polymer processing equipment failure due to formation of agglomerates (ropes, chunks).
This “fouling” is caused in part by fines and also by the build up of electrostatic charge on the walls on the reactor. Attempts to avoid fouling during slurry polymerization have been made by adding an antifouling agent in the polymerization medium. Typically, the antifouling agent acts for example to make the medium more conductive, thus preventing to some extent the formation of electrostatic charge, which is one cause of the build-up of polymer on the wall of the reactor.
U.S. Pat. No. 3,995,097 discloses a process whereby an olefin is polymerized in a hydrocarbon diluent using a catalyst comprising chromium oxide associated with at least one of silica, alumina, zirconia, or thoria. Fouling of the reactor is said to be reduced by adding a composition, which comprises a mixture of aluminium or chromium salts of an alkyl salicylic acid and an alkaline metal alkyl sulphur succinate.
EP 0,005,215 is concerned with a process for polymerizing olefins in a hydrocarbon diluent again using a catalyst comprising calcined chromium compound associated with at least one of silica, alumina, zirconia or thoria or using a catalyst system such as those disclosed in U.S. Pat. Nos. 2,908,671, 3,919,185 and 3,888,835. The process uses an anti-fouling agent comprising a compound containing a sulphonic acid residue. The anti-fouling agent is a composition comprising (a) a polysulphone copolymer (b) a polymeric polyamine, and (c) an oil soluble sulphonic acid. In the Example, the additive product known as Stadis 450 is used as the anti fouling agent.
U.S. Pat. No. 6,022,935 (equivalent to EP 0,803,514) discloses a process for the preparation of polymers of C2-C12 alk-1-ene using a catalyst system containing a metallocene complex. An antistatic agent is used in the process. It is said that in general, all antistatic agents which are suitable for polymerizations may be used. Examples given are salt mixtures comprising calcium salts of medialanic acid and chromium salts of N-stearylanthranilic acid, C12-C22 fatty acid soaps of sulfonic esters of the general formula (RR′)—CHOSO3Me, esters of polyethylene glycols with fatty acids, and polyoxyethylene alkyl ethers.
EP 0,820,474 is concerned with preventing sheeting problems in gas phase reactors in polymerization processes, which comprise at least one loop reactor followed by at least one gas phase reactor. These problems are addressed using a fouling preventive agent that is a mixture of Cr salt of C14-C18 alkyl-salicylic acid, a Ca dialkyl sulphosuccinate and a copolymer of alkylmethacrylate with 2-methyl-5-vinylpyridine in solution in xylene. Chromium-type catalysts, Ziegler-type catalysts and metallocene catalysts are mentioned.
JP 2000-327,707 discloses a slurry olefin polymerization method. The method addresses the problems of fouling and sheeting of the reactor wall, which is observed particularly with supported metallocene catalysts. The method is said to be carried out in the presence of one compound chosen from polyalkylene oxide alkyl ether, alkyl diethanolamine, polyoxyalkylene alkyl amine, and polyalkylene oxide block.
EP 1 316 566 discloses propylene polymerization in a bulk loop reactor. The disclosure is concerned specifically with the transition from one catalyst type to another in a bulk loop reactor and with the problems associated therewith. The process involves injecting a metallocene catalyst and a Ziegler-Natta catalyst system into the bulk loop reactor. There is no disclosure in EP 1316566 of the catalyst being a chromium-oxide type catalyst. It is mentioned on page 3 paragraph [0009] that in one embodiment, a volume of antifouling agent may be introduced into a catalyst mixing system. Three possible antifouling agents are mentioned. The discussion on pages 10 and 11 clearly teach that an antifouling agent is used for the metallocene catalyst systems and not for conventional Ziegler-Natta catalyst systems. Further, the metallocene catalyst and Ziegler-Natta catalyst are injected into the loop reactor sequentially in EP 1 316 566 and not simultaneously so that they are not both present in the reactor at the same time and so that any antifouling agent present in the metallocene catalyst system will not contact the Ziegler-Natta catalyst system.
In view of the above, it will be seen that many so called anti-fouling agents for use in various olefin polymerization processes are known. However, there have been some problems associated with prior known agents, particularly in relation to polymerization processes using chromium-type catalysts and sometimes Ziegler-Natta type catalysts. These problems include an increase of catalyst consumption due to loss of activity in the presence of the anti-fouling agent. This can be observed even at the low levels typically used in the polymerization process. Catalyst activity loss is linked to the poisoning of active sites, for example by the polar moieties of the anti-fouling agent (alcohol and sulphonate . . . ).
Other problems with prior known agents relate to problems of toxicity. This is a particular concern with Cr-based anti-fouling agent or with agents such as commercial Stadis 450 as described in EP 0,005,215, because of the solvent type (toluene) and/or because of the active ingredient.
Finally, practical problems are encountered with many previously known anti-fouling agents. These practical problems arise because some antifouling agents are usable only with a given catalyst type. This makes transitions between catalyst systems during processing more difficult.
Thus, there remains a need to provide new anti-fouling agents for use in olefin polymerization processes using chromium-type catalysts, late Transition Metal-type catalysts, or Ziegler-Natta type catalysts without the drawbacks of current products.
High density polyethylene (HDPE) was first produced by addition polymerization carried out in a liquid that was a solvent for the resulting polymer. That method was rapidly replaced by polymerization under slurry conditions according to Ziegler or Phillips. More specifically slurry polymerization was carried out continuously in a pipe loop reactor. A polymerization effluent is formed which is a slurry of particulate polymer solids suspended in a liquid medium, ordinarily the reaction diluent and unreacted monomer (see for Example U.S. Pat. No. 2,285,721). It is desirable to separate the polymer and the liquid medium comprising an inert diluent and unreacted monomers without exposing the liquid medium to contamination so that said liquid medium can be recycled to the polymerization zone with minimal or no purification. As described in U.S. Pat. No. 3,152,872, a slurry of polymer and the liquid medium is collected in one or more settling legs of the slurry loop reactor from which the slurry is periodically discharged to a flash chamber thus operating in a batch-wise manner.
The mixture is flashed in order to remove the liquid medium from the polymer fluff. It is afterwards necessary to recompress the vaporized polymerization diluent to recondition and purify it.
Due to economical incentives, the reactor is generally pushed to its limits of operability. High concentration of monomer and optional comonomer, high temperature and high solid content are three important factors that allow to increase the kinetics of the polymerization chemical reaction.
The power consumption of the circulation pump normally increases slowly with increasing solid content. When any one of the three parameters just mentioned (monomer and optional comonomer concentration, temperature and solid content) increases above a certain level, depending upon the polymer characteristics and upon the reactor characteristics, it is additionally observed that the level of noise of this power consumption starts increasing gradually and if not properly controlled may provoke the safety shut-down of operations. This behaviour is known as the swelling phenomenon. The same type of behaviour can be observed on other plant control measurements such as, without limitation, the reactor temperature, the slurry density or the temperature change experienced by the cooling water circulating in all or a portion of the cooling jacket.
Monomer concentration and reactor temperature are usually kept nominally constant to maintain the product quality in the narrow specification required. Increasing solids concentration generally improves the product quality as, at constant reactor throughput, the residence time in the reactor, defined as the mass of solids present in the reactor divided by the production, increases with increasing solids concentration.
It is indeed desired to increase the residence time in the reactor in order to maximize the contact time with the catalyst and to improve the granulometry of the final product. As the mass of solids present in the reactor is defined as the product of the reactor volume by the density of the slurry and by the solid content, and as the density of the slurry is increasing with the solid content, it is thus highly desirable to increase the solid content. Unfortunately, the most usual cause of swelling is high solid content.
It is well known that polymers of olefins can be prepared by olefin polymerization in a hydrocarbon diluent or in monomers acting as diluents. However, it has been found on an industrial scale that where the polymer is insoluble or substantially insoluble in the diluent, the polymer product has a tendency to deposit on the wall of the polymerization reactor. This so-called “fouling” leads to a decrease in the efficiency of heat exchange between the reactor bulk and the coolant around the reactor. In some cases, the temperature difference between the reactor bulk temperature and temperature of the coolant (e.g. a cooling water system) can increase over time to a level, which means that the run must be terminated.
This “fouling” is caused by a combination of fines and the build up of electrostatic charge in the powder. Attempts to avoid fouling have been made by adding an antifouling agent to the diluent as a processing aid. Typically, the antifouling agent acts to make the diluent more conductive. This prevents to some extent the formation of electrostatic charge, which is one cause of the build-up of polymer on the wall of the reactor.
U.S. Pat. No. 3,995,097 discloses a process whereby an olefin is polymerized in a hydrocarbon diluent using a catalyst comprising chromium oxide associated with at least one of silica, alumina, zirconia, or thoria. Fouling of the reactor is said to be reduced by adding a composition, which comprises a mixture of aluminium or chromium salts of an alkyl salicylic acid and an alkaline metal alkyl sulphur succinate.
EP 0005215 is concerned with a process for polymerizing olefins in a hydrocarbon diluent again using a catalyst comprising calcined chromium compound associated with at least one of silica, alumina, zirconia or thoria or using a catalyst system such as those disclosed in U.S. Pat. Nos. 2,908,671, 3,919,185 and 3,888,835. The process uses an anti-fouling agent comprising a compound containing a sulphonic acid residue. The anti-fouling agent is a composition comprising (a) a polysulphone copolymer (b) a polymeric polyamine, and (c) an oil soluble sulphonic acid. In the Example, the additive product Stadis 450 is used as the anti fouling agent.
U.S. Pat. No. 6,022,935 (equivalent to EP 0803514) discloses a process for the preparation of polymers of C2-C12 alk-1-ene using a catalyst system containing a metallocene complex. An antistatic agent is used in the process. It is said that in general, all antistatic agents which are suitable for polymerizations may be used. Examples given are salt mixtures comprising calcium salts of medialanic acid and chromium salts of N-stearylanthranilic acid, C12-C22 fatty acid soaps of sulfonic esters of the general formula (RR′)—CHOSO3Me, esters of polyethylene glycols with fatty acids, and polyoxyethylene alkyl ethers.
EP 0820474 is concerned with preventing sheeting problems in gas phase reactors in polymerization processes, which comprise at least one loop reactor followed by at least one gas phase reactor. These problems are addressed using a fouling preventive agent that is a mixture of Cr salt of C14-C18 alkyl-salicylic acid, a Ca dialkyl sulphosuccinate and a copolymer of alkylmethacrylate with 2-methyl-5-vinylpyridine in solution in xylene. Chromium-type catalysts, Ziegler-type catalysts and metallocene catalysts are mentioned.
In view of the above it will be seen that many so called anti-fouling agents for use in olefin polymerization processes are known. However, there has been a problem with prior known agents, particularly in relation to polymerization processes using chromium-type catalysts or Ziegler-Natta type catalysts because of loss of activity of the catalyst due to the presence of the anti-fouling agent. This is because of poisoning of the catalyst, for example by alcohol and sulphonate groups in the anti-fouling agent.
Other problems with prior known agents relate to problems of toxicity. This is a particular concern with Stadis 450 as described in EP 0005215.
Thus, there remains a need to provide new methods for preventing fouling in olefin polymerization processes, especially in the polymerization of ethylene and more especially in the polymerization of high molecular weight polyethylene.
For many years it has been desirable to increase the efficiency of polyolefin production. One goal has been to increase the quantity of polyolefin that can be produced in a given volume of reactor. The higher the quantity that can be produced, the lower the cost of product production, which provides clear market advantages.
One method for increasing the quantity of product produced per unit volume of reactor is to increase the concentration of the monomer in the reactor. Clearly, the greater the concentration of the monomer, the greater the concentration of the final product in the reactor. However, there are a number of problems associated with increasing the monomer concentration, as discussed below.
Generally, polymerization of olefin monomers is an exothermic reaction. The reaction follows first order kinetics. Thus, the higher the monomer concentration, the faster the reaction proceeds, and the greater the quantity of heat that is released by the reaction process. This heat production may be extremely dangerous if it is not controlled. Clearly a build-up of heat in a reactor containing flammable hydrocarbons may lead to fires or explosions.
In order to solve this problem and to use as high a monomer concentration as possible, typically two measures have been taken in the past. Firstly, olefin polymerization reactors have been carefully designed to control the surface area:volume ratio of the reactor. This ensures that there is sufficient surface area to the reaction vessel to allow heat exchange with the outer environment, thus reducing the temperature inside the reactor. Single or double loop reactors are common. These reactors consist of a long pipe, arranged in one or two loops, each loop being tens of meters high. The diameter of the pipes is typically around 60 cm. Such an arrangement has a large surface area:volume ratio as compared with a conventional flask or tank arrangement. Secondly, the reactors are usually jacketed with a cooling system, such as with a water jacket. This serves to efficiently carry away heat from the surface of the reactor, to increase the efficiency of cooling.
However, generally these methods have only been suitable for monomer concentrations of from 4-6.5 wt. %. This is because a further problem exists with increasing monomer concentration. Often the monomer is gaseous at the temperatures and pressures employed in the reaction. At elevated concentrations of the monomer, the monomer may pass out of solution and form pockets of gas in the reactor. This has clear disadvantages. The gas formed can lead to dangerous pressure build-up. In addition, the release of monomer from the solvent reduces the monomer available for reaction, unbalancing the carefully selected concentration of reactants and leading to undesirable products and impurities. This may have the effect of reducing the efficiency of the process rather than increasing it. Finally, the reactants are typically pumped around the reactor loop for efficient mixing and cooling, but the pumps are designed to pump liquids and will not function properly if gas is present.
In a typical polymerization reaction, monomer, diluent, catalyst, co-catalyst and optionally co-monomer and hydrogen are fed to a reactor where the monomer is polymerized. The diluent does not react but is typically utilised to control solids concentration and also to provide a convenient mechanism for introducing the catalyst into the reactor. The reactor effluent, a mixture of polymer, diluent, unreacted (co-)monomer and hydrogen, is removed from the reactor and fed to a flash tank where the polymer is separated from the diluent and unreacted (co-)monomer and hydrogen. Typically, catalyst will be contained in the polymer.
Polymerization processes of ethylene may be carried out in loop reactors. In the polymerization reaction of ethylene, different reactants including the monomer ethylene, a light hydrocarbon diluent such as isobutane, a catalyst and optionally, a co-monomer such as hexene-1 and hydrogen are fed to a reactor. When polymerizing ethylene, in the presence of a suspension of catalyst in diluent, said diluent having low solubility for the polymer, the polymer is produced in the form of solid particles, insoluble in the diluent. The contents of the reactor are circulated continuously with a pump to avoid deposition of polymer on the walls of the reactor. Slurry, consisting of the reactants and polyethylene powder, is typically collected in one or more settlings legs of the polymerization reactor and discharged continuously to a flash tank, through flash lines, where most of the light hydrocarbon diluent and unreacted ethylene evaporates, yielding a dry bed of polyethylene in powder form. The powder is discharged to a purge drier in which the remaining light hydrocarbon and co-monomer are removed. Then the powder of polyethylene is transported to a finishing area where various stabilisers and additives are incorporated. Finally it is extruded into pellets.
For obtaining polymer having suitable properties, it is essential in a polymerization reaction to control the reaction conditions and input component quantities in the reactor. For doing so, it is conventional to sample the reactor contents and control several of the variables of the process in response with the analysis of the sample.
Several methods have been described to take samples from the reactor contents. Generally the reactants in loop-type reactors are propelled at relatively high velocities in order to maintain the catalyst and particulate polymer produced in a suspended state and to prevent deposition or growing of polymer on the reactor walls. It is therefore necessary that no vapor phase is present in the reactor where polymer might grow. In order to take a sample from such reactors, generally a standpipe is placed in the uppermost portion of the reactor to collect slurry. However, the slurry in said standpipe is generally not in equilibrium with the reactants, and hence it is almost entirely impossible to obtain a representative sample.
A vapor sample may be taken from the flash tank. However, sampling of gases from flash tanks has several disadvantages. In polymerization plants using flash tanks which are connected to a reactor by means of flash lines and settling legs, the settling legs themselves can present problems. Conventional settling legs have sections in which polymer can collect while waiting for next dump cycle for transferring the slurry to a flash tank. The collected polymer can melt over time and deposit on the inside walls of the settling leg. In addition, during collection of the slurry in the settling legs and before dumping it to the flash tank, the polymerization reaction still continues. Also, there is a lag in time between recovery of slurry in the settling legs and further processing of the slurry to the flash tank. As a consequence thereof, reaction conditions, which are monitored after transfer of the slurry in the flash tank, are different from the reaction conditions in the reactor. Analysis of a gas sample taken from the flash tank does not provide updated information on the reaction conditions in the polymerization reactor and will result in an inaccurate analysis of the gas composition in the polymerization reactor.
U.S. Pat. No. 3,556,730 refers to a sampling apparatus for taking a sample comprising liquid, dissolved gas and suspended particulate solids from a reactor into a fixed volume chamber. The reaction fluid in the chamber is then rendered non-reactive by immediately adding a predetermined volume of reaction termination fluid. The non-reactive sample is automatically discharged into a separation chamber from which part of the dissolved gas and liquid is continuously analyzed.
U.S. Pat. No. 6,042,790 describes an apparatus and method for maintaining unreacted monomer concentration in a polymerization reactor. In a polymerization process utilising a high pressure flash to separate polymer from unreacted monomer contained in the effluent stream from the reactor, the concentration of unreacted monomer in the reaction effluent is determined by withdrawing from the reactor an effective analyzing amount of effluent, exposing the amount to a low pressure flash and analyzing the vaporised portion to determine the concentration of monomer.
However the above-described devices and methods do not allow the control of several different variables of the polymerization process, such as e.g. monomer, co-monomer and hydrogen in the gas phase and properties of the polymerization product such as the melt flow index and density, in response with the analysis of the sample.
In view hereof, it is clear that there remains a need in the art for providing a more accurate sampling system for taking and analyzing a sample from a polymerization reactor. It is therefore an object of the present invention to provide a device capable of taking out a sample from a polymerization reactor and accurately analyzing said sample. It is further an object of the invention to provide a device capable of taking out a sample from a polymerization reactor, which consists of two reactors being connected in series.
It is another object of the present invention to provide a method for improving a polymerization reaction in a polymerization reactor. In particular, the invention aims to provide a method for improving a polymerization reaction for preparing bimodal polyethylene in a polymerization reactor, which consists of two reactors being connected in series.
U.S. Pat. No. 3,242,150 disclosed an improvement to loop reactors consisting in adding to the bottom part of a loop reactor a receiving zone, since known as settling leg, wherein the solids settle by gravitation, and withdrawing a fraction concentrated in solids from said receiving zone.
U.S. Pat. No. 3,293,000 disclosed a loop reactor with several settling legs. Control of the valve is described at column 3, lines 2 to 22.
U.S. Pat. No. 3,374,211 disclosed a modified process for removing polymer.
More recently, U.S. Pat. No. 5,183,866 related to the employment of a flash line heater in conjunction with the periodic operation of a settling leg of a loop reactor. The process is characterised by the fact that the elongated zone is constructed such that the flow time of the charge of slurry in an elongated confined zone including the flash line heater is equal to at least about 25% of the time between the closing of the settling leg valve and the next opening of the settling leg valve.
Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and catalyst and optionally co-monomers in a loop reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent, the product being often taken off by means of settling legs which operate on a batch principle to recover the product. Settling legs are used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further either transferred to another reactor or discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. This recycling may be done either through recompression and reinjection to the reactor with or without intermediate purification. An important operational cost is linked to this fluid effluent recycling. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized. This technique has enjoyed international success with millions of tons of ethylene polymers being so produced annually.
Optimal behavior of the settling legs is reached when the quantity of recovered polymer is maximized with respect to the amount of fluid effluent that must be recycled, so that the recycling cost may be minimized for a given production rate. Classically, operation of the plant is based on attempting to discharge the same amount of slurry from all settling legs in order to afford equivalent pressure drops when discharging each leg, however this operation may be far from optimal.
Various alternatives to conventional settling legs are known. For example, WO 01/05842 describes an apparatus for removing concentrated slurry from a flowing stream of slurry in a conduit characterized by a channel in an outlet area of the conduit, the outlet being adapted to continuously remove slurry.
EP 0891990 describes an olefin polymerization process wherein the product slurry is recovered by means of a continuous product take off, more in particular by means of an elongated hollow appendage provided on the reactor. Said hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.
However the above-described apparatus and processes have the disadvantage that the suspension withdrawn from the reactor still contains a large amount of diluent and of other reactants, such as the monomer, which it is then necessary to subsequently separate from the polymer particles and to treat for the purpose of reusing it in the reactor.
It is therefore an object of the present invention to provide a polymerization process occurring in a loop reactor wherein discharge of the settled polymer slurry is optimized. Another object of the invention is to provide processes wherein the settling efficiencies of the polymer slurry and its further discharge is optimized. A yet further object of the present invention is to decrease the fluid effluent throughput at a given polymer production rate by the use of optimized discharge. It is another object to provide a loop reactor having optimized settling legs.
The use of slurry-loop reactor systems in the polymerization of olefin monomers is well known in the art. (see for example U.S. Pat. No. 2,285,721). In such system, it is desirable to separate the polymer and the liquid medium comprising an inert diluent and unreacted monomers without exposing the liquid medium to contamination so that said liquid medium can be recycled to the polymerization zone with minimal or no purification. As described in U.S. Pat. No. 3,152,872, a slurry of polymer and the liquid medium is collected in one or more settling legs of the slurry loop reactor from which the slurry is discharged to a flash chamber.
The mixture is flashed in order to remove the liquid medium from the polymer.
For years, those slurry-loop reactors have been operated in a stand-alone configuration for, e.g., the production of monomodal polyolefins.
It has also been known in the art (since for example EP 0 057 420 or EP 0 022 376) that polymerization reactors can be connected in series with, as a result among others, the production of polyolefins with a wide molecular weight distribution, very good homogeneity and outstanding mechanical and processing properties.
The “modality” of a polymer refers to the form of its molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight. If the polymer is produced in a sequential step process, utilizing reactors coupled in series and using different conditions in each reactor, the different fractions produced in the different reactors will have their own molecular weight distribution. It is to be noted that also the chemical compositions of the different fractions may be different.
There however remains the need to be able to produce several kinds of polyolefins such as monomodal or multimodal grades from reactors whether or not connected in series, for different reasons such as a particular need for certain mechanical properties (exclusively obtained either in parallel or in series configuration), for a given residence time, for certain catalyst combination, production issues, equipment availability, plant flexibility, . . . .
Until now, polyolefin manufacturers have been able to manage production of the several kinds of above mentioned polyolefin grades by:
either having dedicated single polymerization reactors on the one hand and dedicated polymerization reactors connected to each other and operated in series, on the other hand,
or being obliged to swing from series to parallel on the same reactor trains.
The first solution is extremely capital intensive.
The second one is extremely time and operation consuming. In this second option, large pieces of connection pipes must indeed be assembled and later disassembled and reassembled again, with all their related utility lines such as flushing lines, heat exchange jackets, measurement and control devices, frame supports, . . . . Connections can often be damaged and the risk for a catastrophic accident is real so that operations are not smooth and are slow down anyway.
A slurry loop reactor run on its own with its settling legs and flash line is already known and does not have to be described here; reference is made, for example, to U.S. Pat. Nos. 3,152,872-A, 3,242,150-A and 4,613,484-A.
Embodiments to operate reactors connected in series are described in details in, for instance, U.S. Pat. Nos. 6,185,349, 4,297,445, EP 0 057 420.
Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and catalyst and optionally co-monomers in a loop reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent, the product being often taken off by means of settling legs which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further either transferred to another reactor or discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. If discharged to a flash tank the polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized. This technique has enjoyed international success with millions of tons of ethylene polymers being so produced annually.
In these polymerization processes, settling legs, however, do present some problems. They represent the imposition of a “batch” or “discontinuous” technique onto a basic continuous process. Each time a settling leg reaches the stage where it “discharges” or “fires” accumulated polymer slurry it causes interferences on the pressure in the loop reactor, which is thereby not kept constant. Pressure fluctuations in the loop reactor may be larger than 1 bar. At very high monomer concentration, such pressure fluctuations may generate several problems such as the creation of gas bubbles that may cause trouble in the operation of the circulation pump. They may also provoke perturbations in the control scheme of the reactor pressure.
Various alternative product removal techniques are however known. For example, WO 01/05842 describes an apparatus for removing concentrated slurry from a flowing stream of slurry in a conduit characterized by a channel in an outlet area of the conduit, the outlet being adapted to continuously remove slurry.
EP 0891990 describes an olefin polymerization process wherein the product slurry is recovered by means of a continuous product take off, more in particular by means of an elongated hollow appendage provided on the reactor. Said hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.
However the above-described apparatus and processes have the disadvantage that the suspension withdrawn from the reactor still contains a large amount of diluent and of other reactants, such as the monomer and optionally the comonomer, which subsequently have to be separated from the polymer particles and to treat for the purpose of reusing it in the reactor. Another disadvantage of the above-described apparatus and processes is their lack of flexibility during the phase or reaction start-up or in response to large disruptions in the normal behavior of the reactor, like sudden interruption of one of the feed streams.
It is therefore an object of the present invention to provide a polymerization process occurring in a double loop reactor wherein the polymer slurry is efficiently removed from the loop reactors through sequentially operated settling legs. It is further an object of the present invention to establish non-fluctuating reaction conditions in a reactor during a polymerization process. More in particular, it is an object of the invention to preserve pressure and to avoid pressure fluctuation in a polymerization reactor. Another object of the present invention is to increase the reactor throughput by providing stable operation conditions. A further object is to increase the monomer concentrations in the liquid medium. Another object of the present invention is to increase the weight percent (wt %) of polymer solids in the polymerization slurry circulating in the polymerization zone in the loop reactors. It is a further object of the invention to provide a flexible process that can be routinely converted to conventional settling leg removal mode in order to adapt to sudden disruption of the operating conditions caused for example by sudden large modification of the diluent or monomer feed throughput rates or start-up conditions.
Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and catalyst and optionally co-monomers and hydrogen in a reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized.
Multiple polyolefin reactors operating in series can be used for olefin polymerizations, as is known in the prior art. Certain polymerization processes comprise the use of two or several polymerization reactors, which are interconnected. A “bimodal olefin polymer” refers to an olefin polymer that is manufactured using two reactors, which are connected to each other in series. However, problems associated with known polymerization processes and apparatuses using a polymerization system having two or more serially disposed polymerization reactor vessels, include inaccurate inter-reactor transfer of polymer slurry between the serially disposed reactors, while maintaining each reactor at independently selected operating conditions. In certain cases, fewer fine particles (fines) are produced during transfer, which tend to hang-up or become trapped in transfer equipment and can even plug lines and valves. Frequent plugging causes system down time, lost final product and raw materials, and increased operating costs.
In the prior art systems, interconnected reactors have been described which are disposed in substantially vertical arrangements, i.e. reactors arranged in tandem vertical arrangement under an angle of inclination with respect to a horizontal axis extending from the exit of the first reactor which is more than 45°. Such arrangements require vertical product transfer lines or other vertical means for transferring polymer product from the polymerization zone of a first reactor to the polymerization zone of a second reactor. However, a problem associated with this type of configuration is that it requires the positioning of the reactors in a vertical arrangement, which is generally technically limited and results in increased fabrication costs. Also in such configurations the reactors are positioned close to one another, which limits their accessibility.
Therefore, there remains a need in the art to provide a method and a polymerization reactor system in which operating problems experienced by prior art multi-reactor systems are reduced and in which the apparatus may be built and operated more economically than prior art systems.
It is therefore a general object of the present invention to provide multiple, interconnected reactors that are built and operated more economically than known prior art multiple reactors. Another object of the invention is to provide an improved method for production of polyolefins in general, and polyethylene in particular, in multiple interconnected reactors. A further object of the present invention is to provide an improved method utilizing multiple, interconnected reactors, which reduces construction and operating costs, and improves operating performance and operating versatility of the reactor system.
Olefin polymerization processes are generally known. Further, it is well known that polymers of olefins can be prepared by olefin polymerization in a hydrocarbon diluent or in monomers acting as diluents. On an industrial scale, one reactor type which may be applied in such processes is a turbulent flow reactor such as a continuous pipe reactor in the form of a loop. However, other types of reactors such as stirred reactors may be used.
Polymerization is carried out in a loop reactor in a circulating turbulent flow. A so-called loop reactor is well known and is described in the Encyclopaedia of Chemical Technology, 3rd edition, vol. 16 page 390. This can produce LLDPE and HDPE resins in the same type of equipment.
The loop reactors may be connected in parallel or in series. In this regard, in a double loop reactor where the two reactors are connected in series, a high molecular weight fraction may be produced in the first loop reactor and a low molecular weight fraction may be produced in the second loop reactor. In this way, a bimodal polymer or a polymer having a broad molecular weight distribution is made. In a double loop reactor where the two reactors are connected in parallel, either a monomodal or a bimodal product is made.
EP0649860, the contents of which are incorporated herein by reference, describes a process for producing polyethylene in two liquid full loop reactors, connected in series.
The ethylene is injected with the comonomer in the first loop reactor as well as the catalytic system (i.e. the catalyst precontacted with the activating agent). Suitable comonomers which can be used include alpha-olefins with from 3 to 10 atoms of carbon, preferably 1-hexene. Polymerization is done at a temperature of between 50 and 120° C., preferably between 60 and 110° C., and at a pressure between 1 and 100 bars, preferably between 30 and 50 bars.
The flow of ethylene polymer obtained in the first reactor is transferred into the second reactor by means of one or more settling legs of the first reactor, for example by using two settling legs (each being filled independently with the suspension coming from the reactor, the solids being concentrated by gravity settling and discharge).
In any olefin polymerization process, the polyolefin is produced in the reactor in the presence of an olefin polymerization catalyst. Such catalysts generally may be classified into three groups: metallocene-type catalysts, chromium-type catalysts and Ziegler-Natta-type catalysts. Typically, the catalyst is used in particulate form. The polyolefin is produced as a resin/powder (often referred to as “fluff”) with a hard catalyst particle at the core of each grain of the powder. The “fluff” is removed from the reactor and must be extruded before it is sold. Typically, an extruder works by melting and homogenizing the “fluff” and then forcing it through holes before cutting to form pellets.
The pellets then may be transformed by subjection to further processing in applications such as pipe making, fibre making, and blow-moulding.
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerization process can be varied in a number of respects to produce a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. In particular, it is known to use polyethylene for use in applications where the polyethylene is required to have crack resistance, both resistance to rapid and to slow crack growth. It is also known to use polyethylene in the manufacture of films where the polyethylene preferably has a low gel content.
It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product can vary depending on what catalytic system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced.
For example, EP-A-0829495, EP-A-946611 and EP-A-946612 all disclose processes for producing polyethylene, these processes comprising copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms, in the presence of a chromium-based catalyst in a first reactor to produce a first polyethylene copolymer product having a first melt index and a first molecular weight distribution, feeding the first polyethylene copolymer product thereby produced and the chromium-based catalyst to a second reactor, and in the second reactor copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms, in the presence of the chromium-based catalyst under different polymerization conditions to produce a second polyethylene copolymer product having a second melt index and a second molecular weight distribution.
In many prior art polymerization processes, the amount of gel is controlled at the expense of throughput: higher residence time is resulting in higher productivity and lower gel content, at the expense of throughput.